Not all inflammation leads to pain. Despite widespread infection followed by fever, colds rarely cause pain. But when some cytokines and certain immune cells are active near pain-sensing nerves, they trigger receptors that convey pain sensations to the brain.

Early on, researchers had learned that the hippocampus was the structure in the brain where long-term memories were created and stored, but it was not known whether the different cell types within this structure might be more or less susceptible to the aging process.